Improved buildings gui, internal structure (#29)

Now it's easier to add buildings to building panels . They are
positioned automatically and also get their name and icon from the
buildingtype.
Major Gui refactoring

The gui is now much more structured and it's much easier to add building
categories.
Improved street deleting (#27)

Streets can now be deleted by dragging over the street you want to
delete.
 On branch Benedikt
 Changes to be committed:
	deleted:    src/game/BufferTools.java
	modified:   src/game/Grid.java
	modified:   src/game/GridCell.java
	modified:   src/game/Main.java
	modified:   src/game/ResourceManager.java
	modified:   src/gui/BuildingPanel.java
	modified:   src/objects/BuildPreview.java
	modified:   src/objects/Entity.java
	new file:   src/objects/Mesh.java
	deleted:    src/objects/Model.java
	modified:   src/objects/ObjectLoader.java
	modified:   src/objects/Streets.java
	modified:   src/objects/Terrain.java
Improved the AnimationManager

The AnimaationManager now accepts custom values and times as parameter.
This is also used for smoother camera animations.
Added custom finished action to AnimationManager

When a custom value is used, a method finishedAction() can be defined,
that is called when the animation finishes.
Merge pull request #34 from ForceField-IndieGames/Benedikt

Gui refactoring, better street deleting, improved AnimationManager